Even the most modern aircraft currently being built are surprisingly inefficient and complicated, making them expensive to design, build, operate, and maintain. Also, even state-of-the-art aircraft are relatively noisy, despite the significant advances that have been made in this area.
One reason that even advanced aircraft have these disadvantages is that they do not take full advantage of the opportunities for simplification and efficiency afforded by state-of-the-art computer technology. Instead of employing a flight control methodology which would take full advantage of present day computer technology, even the most advanced, existing aircraft only employ computers primarily to implement existing and established flight control methodology.
Another drawback of existing flight control methodology is that the center of gravity of a conventional aircraft is located relatively far forward. As a consequence, downwardly acting loads are imposed on the tails of the aircraft. This increases the load which must be supported by the aircraft wing, producing a corresponding decrease in the payload which the aircraft can lift.
Another disadvantage of aircraft with such forwardly located centers of gravity is that the aircraft tends to pitch down when the trailing edge control elements or flaps are lowered to increase the coefficient of lift of the aircraft. This is undesirable because pitch-down decreases lift, thus opposing the lift added by the flaps when they are lowered to increase the lift coefficient. Conversely, the pitch-up resulting when the control segments are deployed upwardly to decrease the lift coefficient also opposes this adjustment by increasing lift.
A related disadvantage is that the trailing edge control members of conventional aircraft are so designed and operated that large loads are imposed on the wing at significant distances from the aircraft body. These loads can cause unwanted wing bending; they can also make rapid pull-ups unsafe.
Existing aircraft also have the disadvantage of a relatively high trim drag coefficient. This is attributable in part to the relatively far forward center of gravity of a conventional airplane. Also, conventional aircraft have a variety of wing-and-tail-mounted control members such as ailerons, flaps, and spoilers with ends which are uncovered when they are deployed. The vortices generated at these ends of the control surfaces at any appreciable speed also contribute significantly to drag.
Also, so that take-off rotation can be achieved with an acceptable utilization of power, relatively low nose gear loadings must be utilized in conventional airplane designs. This may make it difficult to steer the airplane, especially in crosswinds and under other adverse conditions.
I suggested above that complexity is a disadvantage of planned and existing aircraft. This is attributable in significant part to the numerous hydraulic systems and control cables which operate the various flaps, spoilers, ailerons, etc. of a conventional aircraft. Furthermore, aircraft hydraulic systems are notoriously trouble prone and difficult to maintain.
Still other drawbacks of existing aircraft and those now coming into existence are attributable to their configuration and to the materials of which they are constructed.
A plot of the cross-sectional areas of a conventionally designed aircraft is an irregular curve with several sharp peaks. In practical terms, this means that the performance of the aircraft and its handling characteristics will suffer as the aircraft approaches the speed of sound, primarily because of the generation of shock waves.
Even advanced passenger and cargo planes are constructed primarily of aluminum alloys and other conventional structural materials. Consequently, those aircraft do not take advantage of the weight savings that could be realized by optimum utilization of modern structural composites with their high strength-to-weight ratios.
Another disadvantage of conventionally designed aircraft is that they must typically be flown in an extreme nose-up position in order to generate the lift needed to avoid stalling when the airplane is slowed for landing. This is undesirable because the pilot's view is restricted in this nose-up altitude, creating a safety hazard. Or, conversely, if conventional flaps are fully deployed to increase the lift coefficient, the safety margin is again undesirably reduced because only a small change in the angle of attack (or attitude) can cause the aircraft to stall.
Conventional aircraft are also susceptible to gusts and wind shear, and these phenomena have been blamed for a number of fatal crashes. At best, pilot skill and workload requirements may be well beyond what might be considered safe in those and other adverse conditions such as steering the aircraft in cross winds, on slick and other non-optimum surfaces, and in dive recovery, for example.